a) Field of the Invention
The present invention relates to semiconductor wafer vapor treatment for extracting trace substance such as impurities on the surface of a specimen such as a semiconductor wafer, or etching an SiO.sub.2 film on the surface of a semiconductor wafer, or cleaning the surface of a semiconductor wafer.
b) Description of the Related Art
In the field of semiconductor manufacturing, patterns of a large scale semiconductor integrated circuit are becoming very fine. Under such a situation, it is required to highly precisely control the quality of a constitutional element of a semiconductor device such as an oxide film and a nitride film. It is also required to etch an SiO.sub.2 film as a constitutional element of a semiconductor device at a stable speed while maintaining the quality of the SiO.sub.2 film at a high precision.
The characteristic of an insulating film of a semiconductor device is degraded if impurities are mixed in the film. The insulating property of a film is deteriorated if alkali metals or heavy metals are mixed in the film. Impurity ions drift in an insulating film under an electric field, and precipitate on interfaces or defects to thereby lower the device performance, and even the function of an integrated circuit may be lost in some cases.
A problem of impurity contamination is associated with not only an insulating film but also other various cases. In a patterning process of a semiconductor wafer, impurities attached to the surface layer of the wafer invade the inner region of semiconductor by diffusion or drift, lowering the performance of the semiconductor device. Even a very small amount of mixed impurities, which has practically made no trouble, has become significant under advanced technology.
In order to control a very small amount of impurities, it is required to have an ability of detecting such a very small amount of impurities and specify the invasion routes, influences, and the like of impurities. In extracting a very small amount of impurities mixed in an insulating film formed on a semiconductor wafer such as an oxide film and a nitride film, a high purity reagent has been used to dissolve the insulating film.
In extracting impurities by dissolving them into a purifying reagent commercially available, however, it has become difficult to obtain a sufficient sensitivity. Another method of extracting impurities at a high sensitivity has been disclosed in which vapors of purifying reagent are used.
FIGS. 26A, 26B, and 26C are cross sectional views showing the structures of typical impurity extractors using vapor phase dissolution. The impurity extractor shown in FIG. 26A uses natural evaporation of reagent disclosed, for example, in Japanese Patent Laid-open Publication No.60-69531. The impurity extractors shown in FIGS. 26B and 26C use compulsive evaporation of reagent disclosed, for example, in Japanese Patent Laid-open Publication No.60-164330 or in the pre-prints 2C13 of 1992 Annual Meeting of The Japan Society for Analytical Chemistry.
Referring to FIG. 26A, semiconductor wafers 122 to be tested are placed upright in a closed anticorrosion housing 121. An insulating film 123 is formed on the surface of each semiconductor wafer 122. For example, the semiconductor wafer is an Si wafer, and the insulating film 123 is an SiO.sub.2 layer.
An anticorrosion reagent container 124 is placed at a different position in the housing 121, and contains high impurity liquid reagent 125 such as hydrofluoric acid. The inner surface and structure of the container 121 are made of acid resistant material such as tetrafluoroethylene. The top of the reagent container 124 is open so that the liquid reagent 125 can freely vaporize.
The housing 121 is sealed air-tight so that naturally vaporized reagent 126 is closed within the housing 121 and reaches the surface of the insulating film 123 via a perforated partition 129. As a result, the SiO.sub.2 film is etched by hydrofluoric acid vapors. Impurities contained in the SiO.sub.2 film are dissolved in droplets 127 of aqueous solution of hydrofluoric acid.
Since test samples are held upright, the droplets 127 fall down into a droplet reservoir 128. The impurity concentration can be known from sampled droplets by means of, for example, atomic absorption spectrometry.
With this method, the impurity concentration of the reagent vapor 126 naturally vaporized in the reagent container 124 at a room temperature is very low. Accordingly, the measurement background becomes low, enabling to measure the impurity concentration of the test sample insulating film at a high precision. According to the disclosure of the above-cited document, the concentration of Fe for example was detected at the precision of 5.5*10.sup.-10 g/cm.sup.2. The measurement precision has been improved further by the advancement of technology.
This method is highly precise. However, the supply of reagent vapors is small because of natural evaporation of reagent at a room temperature. Therefore, it takes a relatively long time to extract impurities, i.e., to dissolve the SiO.sub.2 film.
The method of forcibly vaporizing reagent as illustrated in FIGS. 26B and 26C has been proposed to shorten an impurity extraction time.
In the method illustrated in FIG. 26B, the vapor pressure is raised by heating reagent. Similar to FIG. 26A, a reagent container 124 and a droplet reservoir 128 are arranged in a housing 121. However, the reagent container 124 is surrounded by a heater 130 and the droplet reservoir 128 is accommodated in a cooling vessel 131.
By heating liquid reagent 125 in the reagent container 124 by the heater 130, the amount of vapors of the reagent 125 increases. The higher the heating temperature, the more the amount of generated reagent vapors. In this way, the amount of reagent vapors supplied to the surfaces of semiconductor wafers 122 can be increased.
Because the outer surface of the droplet reservoir 128 is surrounded by the cooling vessel 131, droplets moving down on the surface of the semiconductor wafers 122 and falling into the droplet reservoir 128 are cooled by the cooling vessel 131, suppressing the re-evaporation of droplets. Therefore, a change in the amount of droplets in the droplet reservoir 128 to be caused by the re-evaporation can be avoided, and in addition, it is possible to prevent droplets in different droplet vessels 128 from being mixed through the re-evaporation and re-condensation.
However, if the reagent container 124 is heated excessively, the temperature of generated reagent vapors 126 becomes high. Under such a condition, although the reagent vapors 126 react with the insulating film 123 of each semiconductor wafer 122, no droplet 127 is formed and the surface of the semiconductor wafer 122 becomes dry. In order to recover dried by-products, it is necessary to dissolve them in liquid or to recover them by other methods. Therefore, the extraction precision relies upon the purity of recovering liquid. To avoid this, the heating temperature at which the reagent 125 is vaporized is limited.
Another method has been proposed in which nitrogen (N.sub.2) gas is supplied to heated reagent to blow reagent vapors to the surface of a semiconductor wafer (Technical Study Report SDM 91-159, the Institute of Electronics, Information, and Communication Engineers of Japan). With this method, reagent reacts with the surface of a semiconductor wafer without generating droplets. By-products on the wafer are recovered by flowing recovering liquid such as pure water and by dissolving them.
This method requires to use recovering liquid having a high purity, in order to obtain a sufficiently high measuring precision, even if the purity of reagent vapors blown on the surface of a semiconductor is made high.
In the method illustrated in FIG. 26C, a carrier gas such as a nitrogen gas is conveyed to two reagent containers 124a and 124b and bubbled in liquid reagents 125a and 125b to supply reagent vapors together with the carrier gas to a subject to be tested. The bubbling process increases the amount of reagent vapors and shorten an extraction time. Depending upon the kind of an insulating film 123, different reagents 125a and 125b may be used and mixed together.
With the methods illustrated in FIGS. 26A to 26C, although impurities can be extracted in a short time, the extraction precision lowers. Heating and bubbling impart high kinetic energy to generated reagent vapors so that not only reagent molecules but also mists (particles) having a diameter over 10 fm are generated. Generally, impurities such as Fe contained in the reagent are encircled by large mists and vaporized.
In other words, the evaporation of reagent through heating and bubbling may possibly loose the high purity which may otherwise be obtained by quiet natural evaporation.
Next, a conventional SiO.sub.2 film etching method used by an IC manufacturing process will be described. The etching is generally performed by a wet process using hydrofluoric acid aqueous solution or a dry process using plasma. These processes are associated, however, with the problems of attachment of particles, metal impurities, and the like. A vapor phase dissolution method using a hydrofluoric acid gas has been expected as a prominent means for solving these problems. The vapor phase dissolution etching method is also effective for cleaning a semiconductor wafer because this method allows an underlie layer to be removed by using high purity etching gas and allows a clean surface to be exposed.
FIG. 27 is a schematic diagram illustrating a basic vapor phase dissolution method. Hydrofluoric acid aqueous solution 141 is contained in a container 140. A subject 142 such as an Si wafer having a SiO.sub.2 film as an underlie layer is disposed near at a nozzle opening of the container 140.
As an N.sub.2 carrier gas is introduced from an N.sub.2 gas supplier to the container 140, a hydrofluoric acid gas and water vapors move upward to the surface of the subject 142 which is placed in a dried N.sub.2 gas atmosphere.
Disclosed in J. Vac. Sci. Technol. A, Vol.7, No.3, 1989, May and June issues is the case of etching an SiO.sub.2 film on the surface of an Si wafer subject 142 by heating it from 25.degree. C. to 60.degree. C. by changing the temperature of hydrofluoric acid aqueous solution 141 from 25.degree. C. to 40.degree. C. in accordance with the method illustrated in FIG. 27. According to this disclosure, the etching speed relies greatly on the temperature of the hydrofluoric acid aqueous solution 141, and rapidly lowers at the high temperature.
As the temperature of the subject 142 rises, the etching speed lowers. This may be reasoned from the temperature rise and evaporation of water acting as a catalysis on the surface of the SiO.sub.2 film. This disclosure reports that the etching speed takes a maximum value at the temperature 25.degree. C. of the hydrofluoric acid aqueous solution 141 and at the temperature of 30.degree. C. of the subject 142.
With this SiO.sub.2 film vapor phase dissolution method, the etching speed takes a maximum value at the temperature 25.degree. C. of the hydrofluoric acid aqueous solution and at the temperature of 30.degree. C. of the subject with an SiO.sub.2 film, unable to increase the etching speed over a certain value.
A vapor phase dissolution apparatus for etching a subject by supplying a hydrofluoric acid gas, dried N.sub.2 gas, and N.sub.2 gas containing water contents to the subject, has been developed by Texas Institute of Technology and FSI Ltd. (merchandise name: EXCALIBUR, hydrofluoric acid anhydride gas process system).
In this apparatus, each gas flow is controlled independently to obtain an optional etching speed. The stability of the etching speed therefore depends on the control precision of gas flow.
In order to control the etching speed stably, it is necessary to highly precisely control the flow of a hydrofluoric acid gas, dried N.sub.2 gas, and N.sub.2 gas containing water contents. It is therefore difficult to control the etching speed stably.
Another apparatus has also been developed wherein vapors generated from hydrofluoric acid aqueous solution at a room temperature is supplied to the surface of a subject by using an N.sub.2 carrier gas (merchandise name: VPV-811-A, manufactured by Dainippon Screen Mfg. Co., LTD. of Japan). In this apparatus, the density of hydrofluoric acid aqueous solution is controlled to change the etching speed. In changing the etching speed, hydrofluoric acid aqueous solution is required to be exchanged, resulting in cumbersome operations.
The above-described vapor phase dissolution method (EXCALIBUR, hydrofluoric acid anhydride gas process system) can execute a process of etching an SiO.sub.2 film, a process of rinsing with pure water, and a process of drying a wafer. A semiconductor wafer can be cleaned by executing these three processes.
FIGS. 28A and 28B are bar graphs showing the concentrations of impurities on the surface of an Si wafer cleaned by using this apparatus. FIG. 28A shows the concentrations of residual Cr, and FIG. 28B shows the concentrations of residual Fe. The abscissa represents the type of samples, i.e., samples of control wafers before washing, samples after etching SiO.sub.2 films, and samples after pure water rinsing and drying.
As seen from the graphs, even after the pure water rinsing and drying, Cr and Fe impurity atoms in the order of 10.sup.11 cm.sup.-2 are resident on the surfaces of wafers. The reason for this may be the re-attachment of removed impurity atoms to the active Si surface which atoms have not been rinsed with pure water.
It is also reported that even if contaminations on the surface are dissolved by blowing hydrofluoric acid vapors to the surface of a wafer, impurity elements such as Cu cannot be recovered at the later pure water rinsing (Technical Study Report SDM 91-159, the Institute of Electronics, Information and Communication Engineers of Japan). According to this report, the reason for this is that even if Cu, and the like can be dissolved, these elements re-attach to the wafer surface because of a low ionization tendency and cannot be recovered by the later pure water rinsing and the like.
With such a conventional wafer cleaning method, dissolved metal elements re-attach to an Si surface and it is difficult to recover them completely.